Making TOFu : Fusion Plasma Neutron Emission Spectrometry with a Fully Digital Data Acquisition System

TOFu (Time-Of-Flight upgrade) is a fully digital data acquisition system based on 1 GSPS, 12 bit digitisers for the TOFOR (Time-Of-Flight spectrometer Optimised for Rate) fusion neutron spectrometer at JET. The system has been assessed, developed and subsequently tested during experimental campaigns at JET. A detailed presentation is provided, describing the electronics setup, as well as solutions to challenges related to time-alignment and synchronisation of the signal lines and digitisers. The system enables kinematic discrimination of spectral background, based on associated time and energy measurements. This technique has been tested with synthetic data, evaluated, and compared to experimental results. The kinematic background discrimination method is shown to provide improvements in signal-to-background ratio of up to 500 % in certain spectral regions. TOFOR is optimised for spectrometry of deuterium-deuterium fusion neutron emission at JET. The primary purpose of TOFu is to enable TOFOR to retain these spectrometric capabilities in the presence of a strong background of high-energy deuterium-tritium fusion emission neutrons, in a forthcoming deuterium-tritium fusion plasma campaign at JET. However, the improvement in signal-to-background ratio also allows for detailed studies of low-intensity spectral components, such as the contribution due to neutrons scattering off the internal wall of the JET tokamak before impinging on the TOFOR sight line. Satisfying experimental results pertaining to this aspect of spectral analysis with TOFu data are shown. Finally, a conceptual backscattering time-of-flight spectrometer, based on deuterated scintillator detectors is presented. The backscattering time-of-flight technique is shown to be able to provide high-resolution spectrometric capabilities of deuterium-tritium fusion plasma neutron emission. Studies with synthetic data are used to demonstrate these capabilities and the effects of the developed background discrimination techniques on deuterium-tritium fusion neutron spectra obtained with the instrument.